1. Field of the Invention
This invention relates to stabilized formaldehyde solutions and a method of stabilizing formaldehyde solutions and in particular it relates to concentrated aqueous solutions stabilized with an interpolymer of vinyl acetate, vinyl acetal and vinyl alcohol and to a method of stabilizing aqueous formaldehyde solutions with the aforesaid interpolymer.
2. Description of the Prior Art
Aqueous solutions of formaldehyde are well-known in the art and various additives have been employed in an attempt to prepare and provide stabilized concentrated solutions of formaldehyde. Currently, when employing 37 percent solutions of formaldehyde in water, generally no stabilizer is necessary since at this concentration little or no formation of para-formaldehyde occurs. When preparing aqueous solutions of formaldehyde in excess of 37 percent such as 50 percent solutions of formaldehyde in water, it is necessary to keep the solution at a temperature of over 60.degree. C. in order to prevent formation of para-formaldehyde. Even then the life of the solution is still somewhat limited to a period of hours. Various stabilizers have been proposed and used to prepare concentrated solutions. One common stabilizer is methanol. However, the actual concentration of formaldehyde in aqueous formaldehyde solutions is still limited when methanol is used as the stabilizer in small quantities and if greater quantities of methanol are employed to stabilize more concentrated solutions of formaldehyde in water, the methanol can be determinental to the preparation of satisfactory formaldehyde type thermosetting resins. Other stabilizers have been proposed but again the quantities necessary are quite large and have an adverse effect on their use in preparing thermosetting condensation resins such as phenol-formaldehyde, melamine-formaldehyde and urea-formaldehyde resins.
Recently, polyvinyl acetals have been investigated as stabilizers for aqueous formaldehyde solutions. The use of water insoluble formalin insoluble polyvinyl acetals as stabilizers has been described in combination with an anionic surface active agent used to stabilize the polyvinyl acetal. However, such surface active agents are undesirable because they can generate foaming problems in the aqueous formaldehyde solution, they can affect the properties of resin solutions derived from the aqueous formaldehyde such as the electrical resistance and moisture resistance and because the stabilizer concentrate of polyvinyl acetal and surfactant in water tends to be corrosive and to require special materials of construction.
A method of stabilizing a concentrated aqueous solution of formaldehyde with polyvinyl formal has been described in which the polyvinyl formal is dissolved in a concentrated solution of formaldehyde in aqueous methanol which is then added to the aqueous formaldehyde solution to stabilize it. In forming the stabilizer concentrate the aqueous formaldehyde and methanol is heated for a few hours at a temperature in the range of 130 to 190.degree. F. Stabilization by this method is undesirable because the stabilizer concentrate tends to evolve irritating vapors of formaldehyde, because of the lengthy heating period and because of the poor storage stability and corrosiveness of the concentrate.